From automotive technology, transfer transmissions of countershaft design are known, which are used as transmission arrangements for utility motor vehicles. Usually, in such transmission arrangements the auxiliary drive output is coupled to one of the countershafts by means of a clutch. In a transfer transmission with two countershafts, owing to the non-uniform torque distribution on the countershafts the auxiliary drive output can only be used in a particular position of the main transmission group. This restricts the number of gears available during the operation of the auxiliary output. Moreover, no gearshifts are possible while the auxiliary output is activated. In addition, the auxiliary output cannot be coupled or engaged, or decoupled or disengaged, while driving.
Known from the document 100 21 761 A1 is a shifting device for a transmission with load distribution to two countershafts with two constant gear-trains. At least one constant serves to transmit torque in the transmission to whichever countershaft is associated with an auxiliary output. The shifting device also comprises a synchronization device. The constants comprise three gearwheels, two of which are arranged on the countershafts while the third gearwheel in arranged on the input shaft or the main shaft of the transmission, axially fixed but able to move radially. The gearwheel of the second constant not arranged on one of the countershafts is fixed in the radial direction in a coupling device, in such manner that the coupling device allows radial play of the gearwheel relative to the shaft on which the gearwheel is arranged. A clutch is provided between the synchronization device and the gearwheel of the second constant not arranged on one of the countershafts. When the torque transmitted by the transmission input shaft to the transmission is transferred, via the shifting device, to the gearwheel of the second constant not mounted on one of the countershafts, then the two tooth flanks of the respectively engaged teeth bear on the tooth flanks of the corresponding meshing gearwheels on the countershafts. Due to the radial fixing of the gearwheel of the second constant in the coupling device, the gearwheel can no longer be displaced radially. The gearwheel of the first constant not arranged on the countershafts, by virtue of its radial mounting, forms a coupling element between the two countershafts and thereby produces a form-interlocked back-coupling of the countershaft to which the auxiliary output is coupled, to the opposite and previously not loaded countershaft. As a result of this back-coupling, the tooth forces of the gearwheel of the second constant are distributed in equal measure to both countershafts and returned again, via the first constant, to the countershaft to which the auxiliary output is connected.
The known shifting device, however, has the serious disadvantage that numerous additional components are required in order to produce the shifting mechanism for load distribution to the main transmission group.